Art: A green beast, we see the impact of its back into the cave ceiling.

5) Common Card #5 – Zyx Scorpion [Gloom Lurker http://community.wizards.com/magicthegathering/wiki/Labs_talk:Gds/gds2/loucksj/feedback/challenge4] @ Cretaure – Horror 1/1(@ can only be paid with colorless mana.) Deathtouch Morph B (You may play this face down as a 2/2 creature for 3. Turn it face up any time for its morph cost.)

Art: The defining features of a scorpion (pinchers, tail) can be seen in the darkness.

10) Premium Mythic Card – Liliana Released 5 (cost) Planeswalker – Liliana 4 +2: During target player's next turn, that player can't attack with more than one creature or cast more than one spell. -2: Each player sacrifices a colored permanent. -9: Target opponent gains an emblem with "At the end of your turn, you lose half your life, rounded up."

Art: Liliana, shrouded in darkness, defined by the glowing purple lines on her body.

14) Rare Card #3 – Tariel, the Undersun 3WW Creature – Elemental 5/5 Prismatic White creatures you control have vigilance, blue creatures you control have flying, red creatures you control have first strike, and green creatures you control have trample.

Art: A very bright elemental floating in the center of a large cavern.

15) Basic Land – Cavern Basic Land(T: Add 1 to your mana pool.)

Art: A large stadium-sized cavern, dimly lit by light from various entrances.

Comments:

On @: An empty colorless symbol with a hard dark outline, and colorless cards with @ in their casting cost have punched-up colorless frame to help them stand out. (Similarly, prismatic cards have a punched-up multicolor frame.) Designing with @ has felt like creating the 6th color, without the difficult backwards-compatibility issues. I imagine @ having access to much of the color pie, but with the general theme of making the opponent's life more difficult - damage that can't be prevented, exiling things, can't be countered, unblockable, shroud, deathtouch, etc.

Ideally @ is as draftable as the other colors in limited, as well as being easily splashed. Colorless instants/sorceries are tricks that might be better saved for set two or three, but I'm presenting it here as an option.

On Cavern: Like the other basic lands, players have access to any number of caverns at tournaments, and can play any number in their deck. I can avoid creating a new basic land type by creating a new rule ~305.6b "A basic land with no basic land types has 'T: add 1 to your mana pool.'" This should avoid some of the rules issues a new type creates. Caverns would be distributed to tournament organizers and through booster packs as a possible basic land. I chose to give player's access to caverns because I didn't want people drafting cards they couldn't cast.